Biodegradable elastomeric polymers have recently attracted much attention in the fields of tissue engineering and implantable drug delivery systems. The elastomeric properties of those polymeric substrates offer many advantages over the rigid and tough polymers. First, elastomeric polymers can be designed to offer resemblance to many of the mechanical characteristics and functions of the body tissues and membranes. Second, they have the ability to recover the mechanical challenges which they are subjected to when implanted in a non-static part of the body. Third, this ability to withstand the deformations and mechanical stress would help in retaining the integrity and the functionality of the implantable device. Fourth, biodegradable elastomers are well suited also in soft tissue engineering, where cells are grown into porous scaffolds (with mechanical stimulation) to generate functional tissues. Finally, elastomeric polymers have the ability to transfer mechanical signals between tissues in the place of their implantation.
Biodegradable elastomers reported in the literature have been synthesized as one of two types: thermoplastic1-3 or thermoset elastomers4-6. Thermoplastics have the advantage of being easily processed by melt processing. However, the crystalline crosslinked hard regions these materials possess provide regions of much slower and heterogenous degradation, with the amorphous regions degrading faster than the crystalline segments and so produce a material with physical and mechanical properties that degrade with time in a non-linear fashion. This heterogenous degradation is undesirable for biomedical uses particularly in the drug delivery applications. On the other hand, although thermoset polymers are not easily fabricated by heat, they outperform thermoplastics in a number of areas, including uniform biodegradation, mechanical properties, chemical resistance, thermal stability, and overall durability. For all the above reasons, thermosets attracted attention for their advantageous properties.
One of the common approaches reported earlier to prepare thermoset elastomers is to first prepare multi-arm star condensation polymers by subjecting biodegradable monomers to ring opening polymerization in the presence of polyols as initiators. Some of the most common biodegradable monomers used in that approach include lactides, ε-caprolactone, glycolides, δ-valerolactone, urethane, para-dioxanone, dioexepanone and trimethylene carbonate. The most commonly used polyol initiators included glycerol, laurylalcohol, pentaerythritol and inositol.2,6-10 The prepared star shaped condensation polymers are then crosslinked using thermal or non-thermal approaches. Some of the thermal crosslinking approaches reported involved the preparation of polyurethanes that contain the 4,4′-methylphenidate diisocyanate which degrades to toxic and carcinogenic products and raises issues of biocompatibility.4,11 Other elastomers were prepared by thermal free-radical curing of terminal methacrylated oligomers which involved the use of incompatible catalysts and solvents.5 Some of the compatibility issues of crosslinkers used were overcome by using bis-lactone crosslinkers.12 These crosslinking agents were previously reported in crosslinking lactides, caprolactone and dioxepanone monomers13-15 and lately in crosslinking star polymers made of ε-caprolactone and dl-lactide using glycerol as initiator.6,16 
Although the elastomers made of ε-caprolactone and dl-lactide polymers can be described as absorbable, the rate of their bio-absorption is so slow that it renders the polymers practically useless for many biomedical applications. This is because the main component of the elastomers, which is polylactide absorbs very slowly in bodily tissue. The other primary component is polycaprolactone which absorbs even slower due to its high crystallinity. In addition, lactide polymerizes much faster than caprolactone at 120° C. and so, when the polymers are made, a segmented copolymer containing long segments of polylactide spaced between segments of polycaprolactone is produced. The segmented structures of the polymers further lowers its bioabsorption rate.16 
One other disadvantage is that polymers prepared from ε-caprolactone and lactides will only be composed of hydrophobic segments that contribute to their long and slow bioabsorption and decreases biocompatibility. It is known that highly hydrophobic polymer surfaces have very high contact angles with water and therefore, they are more susceptible to protein adsorption.17-19 This eventually results in formation of fibrous tissues around the implanted device and provokes accumulation of macrophages and other innate immune components around the implant which will eventually result in the device failure. Fibrous capsule formation and mild to severe inflammatory reactions in some cases were also reported.20,21 On the other hand, the acrylated UV crosslinked version of the same reported polymers involved the use of organic solvents like tetrahydrofuran and dichloromethane to incorporate the drug into the precrosslinked mass. This issue raises the flag with regard to compatibility and even stability of loaded bioactive agents.22 Finally, the preparation steps involved in preparing the above elastomers requires higher heat and it takes at least 3 days to obtain the final preparation.6,23,24 
Another approach to prepare elastomeric polymers was also reported through polycondensation reactions between di and tri carboxylic acids and diols which were further subjected to thermal crosslinking. Elastomers based on citric acid, tartaric acid, sebacic acid monomers were reported earlier.25-29 These elastomers either required long curing times ranging from a few days to weeks with inconsistency in the final physical and mechanical properties or the elastomers prepared were tough and brittle. In addition, high crosslinking temperatures were needed for their crosslinking which restricted their use in drug delivery of thermally sensitive therapeutic agents and other heat sensitive drugs.
There remains a need for biodegradable and biocompatible elastomeric polymers.